Session Initiation Protocol (SIP) is a signaling and call setup protocol defined by the Internet Engineering Task Force (IETF) as Request for Comments (RFC) standard 3261. The SIP protocol is text-based and draws certain design elements from hypertext transfer protocol (HTTP), including encoding rules and status codes. SIP is often used as the signaling protocol for the setup and control of SIP communication sessions. As an example, SIP signaling may be used to establish and control signaling for a simple two-way calling session over Voice over IP (VoIP). As another example, SIP signaling may also be used to control signaling for a multi-party multimedia conference session including multiple media streams.
The SIP protocol may be extended in various ways. One such extension is the SIP REFER method defined in RFC 3515, which provides an extension allowing parties to a session to “refer” recipients to a resource referenced in a SIP REFER request. To allow for the referrer to be notified of the status of the transfer, the SIP REFER method implicitly creates a subscription back to the referrer for the status of a REFER request, without requiring an explicit SIP SUBSCRIBE message. Because the implicit subscription relates to the SIP REFER method, once the referral is complete, the implicit subscription likewise ends.